There is known a projector (projection type image display device) that uses an optical modulation element (micro display) such as a liquid crystal or MEMS to visualize illumination light from a light source and projects an image based on the illumination light on a screen. Some of such projectors use, as its light source, a white light source such as a high-pressure mercury lamp and projects an image on a screen while magnifying an image obtained by illuminating a secondary optical modulation element such as a liquid crystal.
However, a high-intensity discharge lamp such as the high-pressure mercury lamp has a comparatively short life, so that when the high-intensity discharge lamp is used for a projector, the lamp needs to be replaced frequently with new one. Moreover, the use of the high-intensity discharge lamp disadvantageously increases a size of the projector. Moreover, it is unfavorable to use the high-pressure mercury lamp that uses mercury, in terms of environmental burden. To solve such drawbacks, a projector that uses laser light as its light source is proposed. A semiconductor laser has a longer life than the high-pressure mercury lamp and allows size reduction of the entire projector.
The laser light thus expected to be used as a next-generation light source for projector is excellent in linearity, so that it is considered that incidence rate can be enhanced more than an LED. However, in a case where the laser light is used as the light source, speckle noise may generate due to high coherence, disadvantageously making an image difficult to see.
The speckle noise is granular noise generated due to interference of scattering light from a minute irregularity of a surface to be irradiated when coherent laser light is used as a light source. The speckle noise generated in the projector not only causes degradation of image quality but also provides physiologic discomfort to a viewer. To reduce the speckle noise, various attempts, such as to vibrate a diffuser plate through which the laser light passes, to widen a wavelength spectrum of laser, and vibrate a screen itself which is an irradiation target of the laser light are made. As one of such attempts for speckle noise reduction, Patent Document 1 discloses a non-speckle display device that reduces the speckle noise by rotating a light diffusing element through which coherent light passes.